Sensors for sensing the rotational position of axles, shafts, or columns such as steering columns are known in the prior art. Rotation position means the orientation of the shaft plus the number of turns the shaft has rotated from a beginning or reference position. Such sensors generally operate in one of three ways. One manner of operation is to combine an orientation sensor with a mechanical or electrical counter to keep track of the specific turn that the sensor is sensing among a multiplicity of turns. The orientation sensor then provides the relative position within any given turn. The advantage of this type of sensor is that a great multiplicity(within the counter's range) can be accommodated. However, a disadvantage arises from the cost and reliability of the "counter" and signal discontinuities at the turn boundaries where the counter must increment the signal output and the orientation sensor must simultaneously return to its zero degree output. Mechanical counters are subject to wear and exhibit "dither" and/or "backlash" error at the increment point. Electronic counters lose their count upon loss of power unless expensive, non-volatile memory is incorporated in their design.
A second type of sensor is a "single-turn" orientation sensor combined with a reduction gear system to translate multiple turns into a single turn. The gear linkages introduce cumulative, mechanical hysteresis and dither, and are also susceptible to wear. High and costly precision in the mechanical linkages is required to minimize error. This precision, in turn, must be maintained in the moving parts (gears) that are most susceptible to wear. If high accuracy and/or high reliability is required, this type of sensor is not practical.
The third type of sensor is a "linear displacement" sensor which is combined with a mechanical conversion linkage such as a "worm drive" to translate multiple turns into a linear displacement. As with the reduction gear system of the second sensor type, the mechanical linkages introduce cumulative hysteresis and dither, as well as adding susceptibility to wear. High and costly precision is also required in these linkages to minimize error. This precision also must be maintained in the moving parts (gears) that are most susceptible to wear. High accuracy and/or high reliability requirements make this type of sensor impractical as well.
The single-turn or displacement sensors of these three sensor types can be either analog or encoder devices, but in all three types multiple turns are not sensed directly.
In certain applications it is desirable to measure small angular displacements of a shaft with a high degree of resolution. For instance, in a torsion bar that is used to measure torque. A rotational load is applied at one end of the torsion bar and the other end is fixed to a reference. The resulting twisting of the bar due to the rotational load results in a rotational displacement between the two ends. In most practical implementations, the maximum degree of rotation is a few degrees. A one percent of full scale of accuracy would then require a resolution on the order of three minutes rotation.
Present methods of measuring such small angles include rotary potentiometers, planar resolvers (a type of variable transformer) and Hall Effect devices among others. All of these methods have limitations in terms of cost, accuracy and/or reliability.